In recent years, polymeric resins have been used more and more in place of existent glass materials, as usual optical components such as spectacle lenses or transparent plates, as well as material for optical components for opto-electronics, for example, optical components for laser equipment such as optical disc devices for recording sounds, video images and character information. This is because the optical material comprising the polymeric resin, that is, optical resin material, is light in weight, excellent in impact resistance and, further, excellent in fabricability and mass productivity due to easy applicability of molding techniques such as injection molding or extrusion molding, as compared with the glass optical material. Such characteristics are useful for various kinds of optical components as described above and are particularly useful in the case of using the optical resin material for various kinds of members constituting a liquid crystal device which is a main constituent element of a liquid crystal display. The liquid crystal display has been used generally as a display element in various kinds of electronic equipment and, along with generalization of the application use, further reduction in the weight and thickness has been demanded and also for the improvement of strength performance such as impact resistance and such demands can be satisfied by effectively taking advantage of the characteristics of the polymeric optical material.
As described above, the optical resin material has a possibility of providing excellent characteristics as optical components for which a wide variety of application uses for optical components have been expected. However, they are not actually utilized as expected. One of major reasons is that products obtained by applying the molding technique to the optical resin material show considerable birefringence, which may sometimes deteriorate their function as optical components.
Birefringence present in the polymeric resin material has been generally known together with the reasons thereof. FIG. 1 is a view for briefly explaining this. As shown in the figure, a polymeric optical material after a molding step is generally in a state in which a plurality of units (monomers) 1 constituting the polymer chain are bonded in a sterically specific orienting direction. In most of the polymeric materials usually used as the optical material, each of the units 1 has an optical anisotropy regarding refractive Index. Namely, the refractive index npr with respect to a polarization phase component in a direction parallel with the orienting direction is different from the refractive index nvt with respect to a polarization phase component in a direction perpendicular to the orienting direction.
As is well known, the optical anisotropy can be expressed by a index ellipsoid. In FIG. 1, an ellipsoidal mark 2 attached to each bonded unit 1 is in accordance with such expression. For example, in the case of polymethyl methacrylate (PMMA), the refractive index for each unit 1 (methyl methacrylate) is relatively smaller in the orienting direction and relatively larger in a direction perpendicular to the orienting direction. Accordingly, an index ellipsoid 3 in view of a macroscale is elongate as shown in the figure. That is, npr&lt;nvt in the case of polymethyl methacrylate. The difference between both of them: .DELTA.n=npr-nvt is referred to as a birefringence value. Table 1 shows birefringence values with respect to typical optical resin materials.
TABLE 1 Double Refraction Value of Polymeric Resin Matrix Material Birefringence value Material .DELTA.n = npr - nvt Polystyrene -0.100 Polyphenylene oxide +0.210 Polycarbonate +0.106 Polyvinyl chloride +0.027 Polymethyl methacrylate -0.0043 Polyethylene terephthalate +0.105 Polyethylene +0.044
For example, .DELTA.n=-0.0043 for polymethyl methacrylate shown in FIG. 1, and .DELTA.n=-0.100 for polystyrene. Further A n shows a positive value as .DELTA.n=+0.044 for polyethylene. Hereinafter, the birefringence due to orientation of a polymer Is referred to as orientational birefringence, and the direction of the index ellipsoid along a major axis is referred to as orientation birefringence direction. Further, when the sign for .DELTA.n is positive (.DELTA.n&gt;0), it is expressed as "the sign of birefringence is positive", while when the sign for .DELTA.n is negative (.DELTA.n&lt;0), it is expressed as "sign for birefringence is negative".
Such orientation birefringence particularly will cause a problem in an application where the polarization characteristic is important. An example is an optical component in a write/erase type opto-magnetic disc developed in recent years. That is, since a polarization beam is used for a reading beam or a writing beam in the write/erase type opto-magnetic disc, if a double refractive optical element (disc itself, lens or the like) is present in an optical path, it gives an undesired effect on the reading or writing accuracy.
Further, a liquid crystal device is mentioned in which birefringence of members used gives the most significant effect. As is well known, the liquid crystal device has a structure of controlling permission/inhibition of the light by rotating the plane of polarization light by a liquid crystal layer between a polarizer and an analyzer arranged as crossed nicols or parallel nicols. Accordingly, in a liquid display device, birefringence of each member constituting the device gives a significant problem, which hinders a generalized application of the optical resin material to the liquid crystal device.
For the orientation birefringence, various attempts have hitherto been made for eliminating the same, typical examples of which are as shown below.
(1) A method disclosed in the specification of U.S. Pat. No. 4,373,065; directed to obtain a non-birefringence optical resin material by blending two types of polymeric resins which have orientation birefringence with signs opposite to each other and which are compatible to each other completely. PA1 (2) A method as disclosed in Japanese Patent Laid-Open Sho 61-19656; utilizing a method of mixing an aromatic polycarbonate and a specific styrenic copolymer at a ratio within a specified range and utilizing the thus obtained aromatic polycarbonic resin composition. PA1 (3) A method as disclosed in Japanese patent Laid-Open Sho 62-240901; for obtaining a non-birefringence optical resin material comprising a mixture of a polymer mainly comprising aromatic vinyl monomeric units and polyphenylene ether, a block copolymer comprising polymeric portions of both of them or a mixture thereof. PA1 (4) A method as disclosed in Japanese Patent Laid-Open Sho 61-108617 of random copolymerizing, graft polymerizing or block copolymerizing two or more monomers constituting positive and negative constituent units having a main polarity difference of not less than 50.times.10.sup.-25 d by an absolute value. PA1 (5) A method as disclosed in the Journal of "Optics", vol. 20, No. 20, pages 80 (30th)-81 (31th) (issued February 1991); proposed by the present inventor for obtaining a non-birefringence optical resin material by copolymerizing a monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methacrylate (3FMA) or a monomer mixture of methyl methacrylate (MMA) and benzyl methacrylate (BzMA). In summary, monomers having opposite signs for orientation birefringence as a basis for giving a polymer are mixed and copolymerized in this method. PA1 (6) A method of molding so as not to cause orientation in the polymer; which is a method adopted based on experience and for eliminating orientation upon molding, for example, by using casting, conducting extrusion molding at a greatly lowered extruding speed or further using biaxial stretching.
The above-mentioned prior art methods can provide respective results to some extent but still involve not a few insuffucient aspects. For example, in the method (1) of blending the two types of polymeric resins, polymeric resins to be blended have to be in a molten state or a solution state in order to mix them at a high uniformity. Then, even by adopting such means, it is actually extremely difficult to obtain a practical polymeric resin showing entirely uniform and less birefringence. In addition, the polymeric resin blend composition obtained by this method inevitably suffers from light scattering caused by irregularity of the refractive index due to the difference of the refractive index inherent to each of the blended polymeric resins, thereby failing to obtain an optical material of excellent transparency.
Among the methods at or after (2), an optical material of high transparency can be theoretically expected in the method of forming the polymeric resin of less orientation birefringence by random copolymerization. However, since two or more of the monomers as a basis for forming the polymeric resin are mixed for random copolymerization in this method, it is necessary to make the monomer reactivity ratio between the monomers closer to 1. However unfortunately, there is so few combinations of materials capable of satisfying such a condition. Accordingly, it involves a problem that the range for the selection of materials is narrowed and that materials can not be selected optionally with a view point, for example, of physical strength or heat resistance.
Although the proposal (5) described above includes such a combination, the method of using the monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methacrylate (3FMA) involves a problem in that the latter material (3FMA) is extremely expensive.
Further, in each of the methods of copolymerizing the monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methacrylate (3FMA) and the method of copolymerizing the monomer mixture of methyl methacrylate (MMA) and benzyl methacrylate (BzMA), occurrence of the orientation birefringence can not be suppressed unless the mixing ratio of trifluoroethyl methacrylate (3FMA) or benzyl methacrylate (BzMA) to methyl methacrylate is Increased considerably. That is, the mixing ratio required for offsetting the orientation birefringence Is MMA/3FMA=50/50 (wt %/wt %) in the former, and MMA/BzMA=80/20 (wt %/wt %) in the latter. Therefore, the resultant material can not have a characteristic equivalent to PMMA and is inferior to PMMA in view of mechanical characteristics or transparency.
In the method (4) of utilizing the graft copolymerization, it is difficult to previously forecast and control the strength of the orientation birefringence of the synthesized resin in a quantitative manner regarding the combination of monomers used, and it is uncertain whether products capable of sufficiently offsetting the orientation birefringence are obtainable or not until the synthetic reaction is actually conducted. Accordingly, it is difficult to manufaccture products of industrially stable quality.
Further, the method (6) is actually used at present for the production of optical resin material used as the member for the liquid crystal device but it can not help deteriorating fabricability and mass-productivity which are the inherent characteristics of the optical resin material. As a result, it leads to a remarkably Increased cost, which restricts the application range of the optical resin material obtained by this method to the liquid crystal device.
Summarizing the above, each of the methods of the prior art still involve problems in that the method; provides fabrication difficulty and can not obtain highly transparent material (method (1)); suffers from restriction for the selection of the material (methods (2) and (3)); has a drawback in the stability of the products (method (4)); requires increased material cost and tends to deteriorate physical property or transparency (method (5)); and can not utilize high fabricability and productivity which are remarkable merits of the optical resin material (method (6)). The problems hinder generalized applications of the optical resin material.
It is accordingly an object of the present invention to provide a technique capable of providing an optical resin material with high non-birefringence without undergoing restrictive conditions as found in the various methods of the prior art. It is also an object of the present invention to provide a member for a liquid crystal device by utilizing the characteristic of the optical resin material obtained by the technique according to the invention.